1. Field of the Invention
The present invention relates generally to the field concerned with the utilization of solar energy for heating and cooling and, more particularly, to an improved solar air heater.
2. Description of the Prior Art
Because the rapid depletion of conventional sources of energy is resulting in an ever-worsening energy crunch, the use of solar energy is becoming increasingly important as an alternative to such sources as petroleum. This is especially true in the field of heating and cooling and in electric power generation.
Solar collectors which use solar energy to heat a black body absorber plate which, in turn, is used in the heating mode to transfer absorbed energy to a fluid heating medium are becoming an important source of at least part of the heat requirement for homes and other buildings. The prior art is replete with numerous attempts to utilize solar collector panels to heat a variety of fluid media by contacting such media with a solar absorber member. The typical solar collector heat transfer media includes water as a liquid medium and air as a gaseous medium.
The use of solar energy heat transfer systems employing liquid fluids such as water rather than gaseous fluids such as air does enjoy two principal advantages. The higher heat transfer coefficients of the liquid media results in lower temperature differences between the liquid and the absorber panel. Also, because liquids have a much higher heat capacity and smaller volume per unit mass, a physically smaller ducting system from the collector to the storage tank or heating system will suffice.
The above advantages, however, are in many cases far outweighed by the major disadvantages from which a liquid system suffers. Liquid systems are subject to leakage and therefore all elements of this system including the absorber panel, all connecting piping and heat storage area must be of leakproof construction. Also, because many of these systems are basically water-based systems, anti-freeze such as ethylene glycol or the like must be added to the liquid medium to prevent freezing. In addition to the leakproof nature of the entire system, the system must be made of a material which is not subject to corrosion by the liquid heat transfer medium. All these are concerns which are real and expensive or difficult to combat in a liquid system yet are unimportant to an air system. With an air system some leakage can be tolerated and phase changes and corrosion do not present problems. The net result is that the liquid system becomes far more expensive than a typical air system because of the above-mentioned disadvantages.
Insofar as an air system itself is concerned, there are several heat transfer configurations which may be considered. Many prior art systems utilize what is essentially a parallel flow wherein the intake air which is being heated is caused to flow parallel to the surface of the absorber plate as the heat transfer therebetween takes place. An example of the use of air as the heat transfer medium in such a system is found in U.S. Pat. 2,998,005 issued to J. G. Johnson dated Aug. 29, 1961. That patent illustrates and describes, inter alia, a solar collector which may use air as the heat transfer medium. While this and similar configurations are easily constructed and inexpensive, the configuration produces a slow moving laminar boundary layer adjacent the heat transfer surface which detracts from the efficiency of the heat transfer because of the slow moving boundary layer. In order to enhance heat transfer, the channel must be narrowed resulting in a higher pressure drop within the system which, of course, necessitates the use of a higher horsepower air circulation system and lowered overall efficiency.
Another alternative system makes use of finned plates to enhance the heat transfer between the absorber plate and the air by increasing the area of contact therebetween. This arrangement seems to work effectively both from the heat transfer and pressure loss standpoints, however, the finned plate construction is relatively expensive in comparison to flat parallel plate construction.